a) Field of the Invention
This invention relates to a pump torque control system for controlling a variable displacement hydraulic pump upon performing work by an equipment driven by working oil delivered from the variable displacement hydraulic pump.
b) Description of the Related Art
Many of hydraulic work vehicles such as hydraulic shovels carry variable displacement hydraulic pumps (hereinafter simply called "hydraulic pumps") mounted thereon, and drive hydraulic actuators by pressure oil from the hydraulic pumps to perform work as required. A hydraulic circuit usable in such hydraulic work vehicles will be described taking a hydraulic shovel as an example.
FIG. 3 is a hydraulic circuit diagram of the hydraulic shovel. In the diagram, there are shown an engine 1, a throttle lever 1a for designating a target speed (rpm) of the engine 1, and a key switch for commanding a start of the engine 1. The throttle lever 1a is provided with an unillustrated target speed (rpm) generator, which outputs an electrical signal proportional to the target rpm designated through the throttle lever 1a. Also illustrated are a hydraulic pump 2 driven by the engine 1 and a displacement varying means 2a for the hydraulic pump 2, such as a swash plate type means or a bent axis type means (hereinafter represented by the "swash plate type means"). Numeral 3 indicates a regulator for controlling the swash plate type means 2a, which is composed of a hydraulic cylinder 3a for driving the swash plate type means 2a and a horse power control spool 3b and flow rate control spool 3c for controlling drive of the hydraulic cylinder 3a.
Designated at numerals 5 and 6 are a boom cylinder and a flow rate control valve for controlling drive of the boom cylinder 5, respectively. In addition to a hydraulic actuator for the boom cylinder 5, the hydraulic shovel is also provided with hydraulic actuators for an arm cylinder, bucket cylinder, swivelling motor, running motor and the like. Flow control valves are also arranged for these hydraulic actuators. These additional hydraulic actuators and their flow control valves are however omitted in the diagram. Designated at numeral 7 is a center by-pass line, which extends from the hydraulic pump 2 to a tank 8 through the individual flow control valves which are in their neutral positions. Numeral 9 indicates a restrictor arranged in the center by-pass line 7 at a position further downstream of the most downstream flow control valve.
Also shown are a pump delivery pressure sensor 10 for detecting a delivery pressure P.sub.d of the hydraulic pump 2 and a negative control pressure sensor 11 for detecting a pressure (negative control pressure) P.sub.n on an upstream side of the restrictor 9. Designated at numeral 12 is a controller, which is inputted with the target rpm from the throttle lever la, the pump delivery pressure P.sub.d detected by the pump delivery pressure sensor 10 and the negative control pressure P.sub.n detected by the negative control pressure sensor so that a predetermined control signal is obtained. Numeral 13 indicates a solenoid-operated proportional valve which operates in response to an output signal from the controller 12.
When any one of the flow control valves, for example, the flow control valve 6 is operated in the above-described construction, a small stroke of the flow control valve leads to a high flow rate through the center by-pass line 7 and a high negative control pressure P.sub.n whereas a large stroke of the flow control valve results in a low negative control pressure P.sub.n. When the negative control pressure P.sub.n is high, the controller 12 computes a target tilting of the swash plate type means 2a so that the delivery pressure of the hydraulic pump 2 is decreased. When the negative control pressure P.sub.n is low, on the other hand, the controller 12 computes a target tilting of the swash plate type means 2a so that the delivery pressure of the hydraulic pump 2 is increased. As a result of this computation, a hydraulic horse power is determined, and the controller 12 performs horse power control by computing the target tilting in such a way that the hydraulic horse power will not exceed the horse power of the engine 1. Described specifically, the controller 12 converts the results of the computation into a command value of electric current for driving the solenoid-operated proportional valve 13, an electric current is outputted from an unillustrated power supply in response to the command value, and corresponding to the electric current, the regulator 3 then drives the swash plate type means 2a. As a result, pressure oil sufficient to drive the boom cylinder 5 is delivered from the hydraulic pump 2. The so-delivered pressure oil produces hydraulic horse power.
On the other hand, rotation of the engine 1 produces a torque on a shaft which is connected to the hydraulic pump 2. Engine horse power is determined by the torque produced here and the rpm of the engine 1. Accordingly, the engine 1 must produce engine horse power sufficient to produce the above-mentioned hydraulic horse power observing this from the side of the hydraulic pump 2, the controller 12 must control the target tilting so that no hydraulic horse power greater than the engine horse power be allowed to occur.
FIG. 4 is a diagram for describing functions which relate to the horse power control by the controller 12. The diagram shows a target rpm generator 1a, for outputting a target rpm corresponding to a stroke of the throttle lever 1a when the throttle lever 1a is operated, and also the pump delivery pressure sensor 10 which is depicted in FIG. 3 and outputs a delivery pressure P.sub.d of the hydraulic pump 2. Also illustrated are a functional computing function unit 121 for storing a data map of engine torques T versus target rpms N (base torque) and determining an engine torque corresponding to a given target rpm, a constant setting function unit 122, a multiplying function unit 123, a dividing function unit 124, and another functional computing function unit 125 for storing a data map of displacements q of the hydraulic pump 2 as obtainable at the dividing function unit 124 versus target tiltings .theta..sub.1 and determining a displacement corresponding to a given target tilting. There are also depicted a further functional computing function unit 126 for being inputted with a negative control pressure P.sub.n and determining another target tilting .theta..sub.2 corresponding to the negative control pressure P.sub.n, and a minimum value selecting function unit 127 for selecting the smaller one of the target tilting .theta..sub.1 obtained by the functional computing function unit 125 and the target tilting .theta..sub.2 obtained by the functional computing function unit 126. Sign .theta..sub.13 indicates the so-selected target tilting which is outputted to the solenoid-operated proportional valve 13.
When a target rpm N is inputted, a corresponding engine torque T is determined by the functional computing function unit 121. At the multiplying function unit 123, the thus-determined engine torque T is multiplied by a constant which has been preset at the constant setting function unit 122. At the dividing function unit 124, the results of the computation are divided by an inputted pump delivery pressure P.sub.d so that a displacement q is obtained. A target tilting .theta..sub.1 of the swash plate type means 2a, which corresponds to the displacement q, is determined at the functional computing function unit 125. At the minimum value selecting function unit 127, this target tilting .theta..sub.1 is compared with another target tilting .theta..sub.2 which has been obtained by the functional computing function unit 126. The smaller one is then outputted as a final target tilting .theta..sub.13 to the solenoid-operated proportional valve 13, thereby making it possible to perform control (horse power control) so that hydraulic horse power will not exceed the engine horse power.
The functions until the displacement q is obtained can be expressed by equations as will be described next.
Assuming now that the mechanical efficiency and volumetric efficiency of the hydraulic pump 2 are .eta..sub.m and .eta..sub.V, respectively, the overall efficiency .eta..sub.p of the hydraulic pump 2 is expressed by: EQU .eta..sub.p =.eta..sub.m .times..eta..sub.V ( 1)
Here, assuming that engine horse power and hydraulic horse power are L.sub.E and L.sub.p, respectively, the following equation can be derived: EQU L.sub.p =.eta..sub.p .times.L.sub.E ( 2)
Further, representing a flow rate of the hydraulic pump 2 by Q, an engine torque by T as described above, and a delivery pressure of the hydraulic pump 2 by P.sub.d as described above, L.sub.E and L.sub.p can be defined as follows: EQU L.sub.E =N.times.T/716 (3) EQU L.sub.p =P.sub.d .times.Q/450 (4)
From the equations (2), (3) and (4), the following equation can be derived: EQU T=P.sub.d .times.Q/(0.628.times..eta..sub.p .times.N) (5)
Assuming that the displacement of the hydraulic pump 2 is q as described above, the flow rate Q in the equation (5) can be defined as follow: EQU Q=.eta..sub.V .times.N.times.q/1000 (6)
Introduction of the equation (6) into the equation (5) makes it possible to determine the displacement q by the following equation: EQU q=628.times..eta..sub.m .times.T/P.sub.d ( 7)
In the equation (7), the engine torque T is the value obtained by the functional computing function unit 121 while the coefficient (628.times..eta..sub.nm) is the value preset at the constant setting function unit 122.
When the controller 12 is composed of a computer, the functional computing function unit 121 stores a data map of engine torques T versus target rpms N at a predetermined area of a memory and extracts an engine torque (base torque) T corresponding a given target rpm N, the functional computing function unit 125 stores a data map of displacements q versus target tiltings .eta..sub.1 at another predetermined area of the memory and extracts a target tilting .eta..sub.1 corresponding to a given displacement .eta..sub.1 and likewise, the functional computing function unit 126 stores a data map of negative control pressures P.sub.n versus target tiltings .eta..sub.2 at a further predetermined area of the memory and extracts a target tilting .eta..sub.2 corresponding to a given negative control pressure P.sub.n. Further, the constant setting function unit 122 stores a constant at a predetermined area of the memory other than the above-mentioned areas. On the other hand, the multiplying function unit 123, dividing function unit 124 and minimum value selecting function unit 127 are computing function units which are generally included in a computer.
Appropriate horse power control is performed by the controller 12 as described above. However, if the engine has been used over an extended period of time or fuel for the engine is not of good quality, the above-described horse power control may often cause stalling during work. In such a case, the work cannot be performed smoothly. If a new engine is used, an engine torque greater than a predetermined engine torque may conversely be produced. In this case, the above-described horse power control may cause a situation where the useful large engine torque cannot be effectively used.